This project is a component of our ongoing study, in collaboration with investigators from the University of British Columbia, of physiological adaptation to the hypoxia encountered at high altitudes. Previous studies had shown changes in the energy metabolism in brain and heart in chronically exposed native populations. The purpose of this study was to determine the effects following short-term exposure to altitudes of 18,000 ft. or more. Standard positron tomographic deoxyglucose measurements were performed in heart and brain in six highly trained individuals before and immediately following military training on Mt. McKinley.